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Space, time and the nervous system Romi Nijhawan University of Sussex Summary of the talk • Philosophy; Neutral monism; the flip-flop analogy • Basics of visual processing. Time delays in the transmission of neural signals • Neutral monism and the Flash-lag effect • Neutral monism and the life-span development of the scientist (from zygote embryo fetus newborn adult) Neutral monism • Holds that ultimate reality is of one kind • Intrinsic nature of ultimate reality is neither mental nor physical; it is neutral between the two – – – – Baruch Spinoza (1632-1677) Ernst Mach (1838-1916) William James (1842-1910) Bertrand Russell (1872-1970) Bodies do not produce sensations, but complexes of sensations (complexes of elements) make up bodies. If, to the physicist, bodies appear the real, abiding existences, whilst sensations are regarded merely as their evanescent, transitory show, the physicist forgets, in the assumption of such a view, that all bodies are but thought-symbols for complexes of sensations (complexes of elements). - Mach (1897) Thus the great gulf between physical and psychological research persists only when we acquiesce in our habitual stereotyped conceptions. A color is a physical object as soon as we consider its dependence, for instance, upon its luminous source, upon other colors, upon temperatures, upon spaces, and so forth. When we consider, however, its dependence upon the retina…it is a psychological object, a sensation. Not the subject matter, but the direction of investigation, is different in the two domains. - Mach (1886) The data of psychology do not differ in their intrinsic character from the data of physics. I have maintained that sensations are data for psychology and physics equally… - Russell (1921) Common sense imagines that when it sees a table it sees a table. This is a gross delusion. When common sense sees a table, certain light waves reach its eyes, and these are of a sort which, in its previous experience, has been associated with certain sensations of touch, as well as other people’s testimony that they also saw the table… The light waves caused occurrences in our eyes, and these caused occurrences in the optic nerve, and these in turn caused occurrences in the brain… (Of course, if matter in general is to be interpreted as a group of occurrences, this must also apply to the eye, the optic nerve, and the brain). - Russell (1959) The modern would-be materialist thus finds himself in a curious position, for, while he may with a certain degree of success reduce the activities of the mind to those of the body, he cannot explain away the fact that the body itself is merely a convenient concept invented by the mind. We find ourselves thus going round and round in a circle: mind is an emanation of body, and body is an invention of mind. Evidently this cannot be quite right, and we have to look for something that is neither mind nor body, out which both can spring. -Russell (1929) Neutral monism: an analogy (In the analogy Flip and Flop represent opposite extremes of a dualistic philosophy) Flop Flip • Real • Studied by physicists • Relevant to psychologists and neuroscientists as a stimulus only • Well known proponents: Newton, Einstein • Perceived • Studied by psychologists and neuroscientists • Relevant to physicists only as a source of intuitions about ‘forces’ and other quantities; or in interpretation of data (including design of measuring devices) • Well known proponent: Berkeley In the analogy a move toward neutral monism Flipflop Neural delays in the retina LIGHT Receptor hyperpolarization Delays due to signal transmission from retina to primary visual cortex and beyond Neural delays in response to a discrete visual stimulus Neural delays in processing of a continuous (moving) visual stimulus Flop? Perceived Flip? Real • Assume: • Object velocity = v • Neural processing Delay= t “vt-lead premise”: • Due to neural delays a moving object’s real position should lead its perceived position by vt. • Dilemmas and paradoxes • Phenomenon: Flash-lag effect On the ‘real position = perceived position + vt’ premise • Real and perceived are spatially separated for motion but not for stationary objects (i.e. when v = 0) • For stationary objects, are the real and the perceived one and the same, or are the real and the perceived co-localized (congruent)? stationary object Flash-lag effect Dark lab, two light sources and a bar painted white. Light source 1 Light source 2 What do observers see at the instant of the flash? Percept F M F “Flash-lag effect” Movement direction demo What does the flash-lag effect have to do with neutral monism? “I see, therefore, no opposition of the physical and the psychical, no duality, but simply identity.” -Ernst Mach, 1890 Mach denied quantities such as absolute space and absolute time due to the fact that these quantities were not observable “This book exercised a profound influence upon me… while I was a student.” --Albert Einstein •Can the spatial lag between the perceived and the real position of the moving object be observed (measured)? •Is this observation possible even in principle? NO! A B midpoint 01:004 Following Mach: If it is unobservable it does not exist A B midpoint 01:004 On the ‘real position = perceived position + vt’ premise the following is a correct picture But this assumption leads to contradiction Consider an object moving at Velocity = v v sensory delay = t vt R E A L Perception trails behind 20 - 160 ms FLE Flash source (off) vt 0 clock 0 FLE vt 0 FLE vt 0 FLE vt Either the above picture is completely wrong. OR It is partly wrong and there are two different t’s: tm and tf, with tm < tf No evidence for faster processing of motion 280 RT FLASH 270 RT MOTION 260 250 240 230 0 1.17 2.34 3.51 4.68 Velocity (deg/sec) Nijhawan et al. (Visual Cognition, in press) Lead of flash for perceived simultaneity (ms) No evidence for faster processing of motion 25 15 5 -5 -15 -25 1.17 2.34 Velocity (deg/sec) 4.68 Conclusion: The above picture is wrong and neutral monism is correct Intuition and analysis suggest very different answers concerning the functioning of the central nervous system. •Intuition suggests that there is a sharp boundary, and a wide gap, between the sensory and motor functions of the CNS. •Intuition also suggests that there are “real” objects out there in the world which are “perceived” by the observer. •A close look at the brain reveals that more than 95% of neurons in our CNS can neither be categorized as “sensory” nor as “motor”. •The nervous system development starts out with both sensory (touch-proprioception) and motor functions emerging from the same cells. •Many neurons in the somatosensory cortex project to the motor neurons in the spinal cord. •Many neurons in the premotor cortex respond to sensory stimulation. The main divide between the sensory and motor functions of the CNS seems to be centered on the following intuition: Sensory processes are geared to processing objects in the “real” world, while motor processes seem geared to moving limbs etc. of the body to which these processes are attached. The INPUT vs OUTPUT divide The INPUT vs OUTPUT divide has, I believe, kept scientists from accepting that sensory and motor processes are two facets of one underlying process. Factors maintaining the INPUT vs OUTPUT divide: •OUTPUT: The brain “generates” commands that move limbs. •INPUT: Science is dominated by sighted individuals, so the concept of “real objects” is synonymous with “objects out there”. If objects are “out there” then information must travel from objects to our eyes. Hence the concept of INPUT. Here is the dilemma: Talking about input in the absence of what it is an input of is meaningless. If we talk of an object as the source of the input to the visual system (say), then we are using the OUTPUT of the brain —“the object” —to call it the source of the input. Thus, when we use the terms ‘sensory INPUT’ in the traditional sense, we are confusing OUTPUT with INPUT. I argue that “real” objects are actually a result of one branch (branch B) of the output of the CNS. The other branch of the output (branch A) is dominated by touch, proprioception and the motor functions. Branch A is earlier both in phylogeny and ontogeny than branch B. Thus both sensory and motor functions are outputs. Speculations based on evolution, development and plasticity The human nervous system development Human development: 10 - 41 weeks 41 weeks … the possibility of learning the significance of the local signs which belong to our sensations of sight, so as to be able to recognize the actual relations which they denote, depends, first, on our having moveable parts of our own body within sight. -- Helmholtz Functional significance of arm movements in neonates 10 - 24 days old A. van der Meer, F. van der Weel & D.N. Lee, Science, 1995 Motor flash-lag Voluntary movement of a rod in the absence of visual feedback (Nijhawan and Kirschfeld, 2003) QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. The outcome sensed position of rod Flash-lag Effect for Voluntary Limb Movement Flash-lag Effect (cm) 12 10 8 6 4 2 0 PM RJ ME Observer MS Motor Flash-lag anisotropy Motor task fixation point Motor task fixation point Motor task fixation point Motor task fixation point Vision task Flash fixation point Flash Vision task fixation point Vision task fixation point Vision task Flash-lag effect (cm) Flash-lag effect (cm) Motor task 12 12 10 10 8 6 4 2 Movement toward fixation Movement away from fixation 8 6 4 2 Movement toward fixation Movement away from fixation vision + touch neurons in area F4 of premotor cortex --Graziano, Hu and Gross, 1997 The visual receptive fields are anchored to a given body part, and move with the body part irrespective of eye position Many neurons in area F5 of premotor cortex respond when the animal performs a particular action, and to visual stimulation in the absence of overt action. -- Rizzolati, Fadiga, Gallese and Fogassi, 1995 The intermediate system “controls” the kinematics of limb movements and of “external” visual objects. Intermediate net Limb movement Peripheral input Object motion Peripheral input